页岩气井压后返排规律

Flowback patterns of fractured shale gas wells

岩气藏通常都需要进行大规模的水力压裂才具有工业开采价值,但是页岩气井压后返排率普遍较低。针对这一问题,采用数值模拟和实验相结合的方法,研究了天然裂缝间距、裂缝导流能力、压裂规模、压力系数和关井时间等因素对返排的影响,并从机理上分析了页岩气井压后返排困难的原因。结果表明:返排率随天然裂缝间距、裂缝导流能力和压力系数的增加而增加,随压裂规模和关井时间的增加而减少;从微观机理进行分析,水通过毛细管自吸作用进入微裂纹,页岩基质中矿物颗粒间原有的氢键被羟基取代进而发生水化作用,造成新的微裂纹的产生和主裂缝的扩展,形成复杂的裂缝网络,使得大部分水难以返排,返排率低;对于页岩气井压裂,一般裂缝间距和裂缝导流能力较小、压裂规模很大,很大一部分注入水存在于比表面积极大、形态极为复杂的裂缝网络中,以致无法返排。结论认为:页岩气井压后返排率的高低受多种因素的影响,不应该刻意追求返排率;低返排率的页岩气井的产量一般较高。

Shale gas reservoirs generally need to be fractured to reach the industrial production. However, the flowback ratio of frac tured shale gas wells is low. In view of this, the effects of natural fracture spacing, fracture conductivity, fracturing scale, pressure coefficient and shut-in time on the flowback ratio were examined by a joint method of numerical simulation and experiments, and the causes of flowback difficulty of shale gas wells were analyzed. The results show that the flowback ratio increases with the increase of natural fracture spacing, fracture conductivity and pressure coefficient and decreases with the increase of fracturing scale and shut in time. From the perspective of microscopic mechanism, when water enters the micro-crack of the matrix through the capillary self-ab- sorbing effect, the hydrogen bonds between the particles are replaced by the hydroxyl group, namely, hydration effect, giving rise to the growth o{ new micro-cracks and the propagation of the main fractures, and complex fracture networks are generated, so a large proportion of water cannot he flowed back, resulting in a low flowback ratio. In shale gas well fracturing, generally there is small fracture space, fracture conductivity and big fracturing volume, so a large proportion of the injected water will be held in the very complex fracture network with a big specific area, and is unable to flow back. It is concluded that the flowback ratio of fractured shale gas wells is affected by several factors,but deliberately seeking to a high flowback ratio is not necessary,because low flowback shale gas wells , instead, usually have high production.